1. Technical Field
The present invention relates to a method of surface-finishing a glass substrate for a magnetic disk used in a hard disk drive and relates to a glass substrate for a magnetic disk.
2. Background Art
Patent Document 1 (Japanese Unexamined Patent Application Publication No. 7-230621) discloses that attention is focused on the fact that one of the factors governing the mechanical strength of a glass substrate for a magnetic disk is flaws present on the surface of the inner peripheral edge of the substrate where the maximum tensile stress will occur during the operation of the magnetic disk, and that at least the end face of the inner periphery is etched to achieve a predetermined surface roughness. The glass substrate described in Patent Document 1 has the surface of the inner peripheral edge or the outer peripheral edge such that, as measured with a three-dimensional SEM at least randomly selected four places with a reference length of 240 μm and a cut-off wavelength of Ra of 80 μm, the mean value of Ra is in the range of 1.0 to 6.0 μm and the mean value of the number of peaks is in the range of 8 to 30.
Patent Document 2 (Japanese Patent No. 3527075) discloses that a glass substrate has a predetermined surface roughness in order to prevent the generation of particles that cause thermal asperity on the glass substrate for a magnetic disk. This document is not aimed directly at an increase in the mechanical strength of the glass substrate. The glass substrate has a surface roughness Ra of less than 1 μm and an Rmax of 0.010 to 4 μm (10 to 4,000 nm) at the inner peripheral edge (chamfer and side wall).
Patent Document 2 describes “edges of the glass substrate subjected to chemical etching (chemical polishing) have satin-finished surfaces. Edge treatment, e.g., chemical etching, that has been performed in the past is not sufficient . . . (snip) . . . . Chemical etching is disadvantageously liable to lead to misalignment of the axes of the inner peripheral edge and the outer peripheral edge. Also in this point, subjecting the edges of the glass substrate to chemical etching is disadvantageous.” in paragraph [0010]. That is, chemical polishing is problematic.
An example of a method of improving the mechanical strength of a glass substrate for a magnetic disk is chemical strengthening treatment. For example, chemical strengthening treatment includes immersing a glass substrate in a molten salt bath mainly containing a potassium nitrate salt heated at a high temperature such as about 400° C. to 500° C. for about 0.5 to 12 hours to perform ion exchange between Na ions in the surface layer of the glass substrate and K ions in the potassium nitrate. In other words, Na ions having a small ionic radius in glass are replaced with K ions having a large ionic radius, thereby resulting in compressive stress being applied to the uppermost layer to reinforce the substrate. A magnetic film for magnetic recording is formed on a main surface of the glass substrate for the magnetic disk. The magnetic film is disadvantageously corroded by K ions concentrated in the surface layer of the glass substrate by chemical strengthening treatment. Furthermore, chemical strengthening treatment requires considerable time and cost.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 7-230621    [Patent Document 2] Japanese Patent No. 3527075
Hitherto, mechanical strength has been known to tend to increase with increasing etch depth (depth of chemical polishing). With respect to the amount etched (polishing depth of the surface layer of chemical polishing), for example, Patent Document 1 discloses that the etch depth is in the range of about 5 to 25 μm and preferably about 10 to 30 μm (paragraph [0039]). Hitherto, thus, to obtain sufficient mechanical strength, an etch depth of 10 μm or more has been necessary.
An etch depth of 10 μm or more disadvantageously reduces finished dimensional accuracy, thus causing nonuniformity in roundness. Hence, the etch depth is preferably less than 5 μm. An etch depth of less than 5 μm eliminates the problems of the decrease in finished dimensional accuracy and the nonuniformity in roundness due to etching.